[markom]

Many pdf books/documents are formated as A5 or two-columns A4, so should be easily read (in terms of font size) on 6" screen in landscape mode (zoomed without margins; two/three screens per page) and using two-column mode (four screens per page).

For reading A4 one-column documents comfortably we need 10" screen, wherewith we can even get slight magnification of paper document, again, only if we read in landscape (zoomed without margins) two-screens per page.

I'd still rather read on 10" e-ink with a good frontlight (landscape; two screens per page), then on 13" without frontlight, because I usually read indoors, with the dimmed light rather than strong.